Preparation of diolefins



Patented Mar. 7, 1 944 Joy G. Lichty, Stow, Ohio, assignor to Wingioot Corporation, Akron, Ohio, a corporation of Delaware No Drawing. Application June 26, 1940,

, Serial No. 342,590

8 Claims. (Cl. 260 -680) This invention relates to a method for the preparation of dioleflns from pentenes and, more particularly, for the preparation of penta dienes from pentenes.

It has been found that pentenes can be broken down by pyrolytic decomposition to yield penta dienes and other conjugated diolefins by the removal of two atoms of hydrogen. Thereaction involved may be illustrated by the following equation for the formation of isoprene from trimethyl ethylene:

C=CH+CH1 CHz=C-CH=OH: H:

Any of the pentenes may be thus treated to term the penta dienes. The pentenes include the following:

n-Propyl ethylene, crnonrorn-cn=cm .9 0. 39 Pentene-2, CHs-CHz-CH=CH-CH: 0.. 36.4 2-methyl-butene-l, cm-cmc=on, .0- 32 CH: Trimethyl ethylene, C=CHCHa C.- 38.4 C I CH: Isopropyl ethylene, cH-cn=cm 0.. 20.1

From these various pentenes may be prepared one or more of the following penta dienes:

Other cmjugated dioleflns. such as other butadieneamayalsobeobtained.

However, it is not possible merely to subject ll tend to yield larger quantities those pentadithe pentenes to a high degree of heat in order to obtain the penta dienes since, as will be observed by inspection of the data given above, the boiling points .of the penta dienes do not in general differ from the boiling points of the pentenes by an amount great enough to permit ready separation of the product from unchanged starting material. Particularly is this true of isoprene and trimethy1 ethylene. In order to overcome this difliculty of separation, the present invention contemplates pyrolysis of the pentenes to such an extent as to convert all or substantially all or the starting material to other compounds with,

,of course, as high a proportion of penta dienes in the product as possible. When the pyrolysis is conducted in this manner, there will be no problem of separation of unconverted pentene or pentenes from the penta diene or penta dienes which are. desired. This resultsfrom the fact that the decomposition products, other than the penta dienes, have boiling points which differ from the boiling points of the pentadienes by a substantial amount, the difference being, on

the average, larger than that which separates the boiling points of the pentenes and the penta dienes respectively. Some of these decomposition products are hydrogen, methane, ethylene and V propylene, all of which boil much lower than do the five-carbon compounds.

Generally speaking, thepyrolysis' may be conducted at a temperature ranging between 700 and 1400 C., the preferred range within which best results are obtained, being 900 and 1100 C.

. The starting material should be subjected to .this

temperature for a period between 0.5 and .03 second,'but usually the contact time will range between 0.2 and .05second. The pressure should be less than atmospheric, pressures corresponding to a range of 10-60 mm./Hg having been found practical, but, obviously the pressure as well as temperature and contact time, may be varied somewhat from the limits given depending on' the nature or the starting material and the results desired. Thatis to say, if it is desired to obtain a particular diolefln with clean separationfrom the other products by pyrolysis, one set of conditions may be employed, whereas it alarger yield of mixed conjugated dioleflns is desired, an.-

other set of conditions may be more efiective.

The method of the invention may also be used to prepare individual pentadienes, such as isoprene trom trimethyl ethylene. In general, where it is desired to secure any given pentadiene, it will be found that the branched chain pentenes enes which are likewise characterized by a branched chain. For example, trimethyl ethylene, unsym. methyl ethyl ethylene and isopropyl ethylene may be expected to yield larger quantitles of isoprene and oi. 2-methyl-butadiene 2:3, while N-propyl ethylene and sym. methyl ethyl ethylene may be expected to produce larger proportions of piperylene sym. dimethyl allene, penta diene 1:4 and ethyl allene. Of course, no clear but division can be made since considerable rearrangement occurs in the pyrolysis of these unsaturated compounds and varying amounts of a' material may be placed in the tube so as to adjust the free space to the'desired volume. Heat comprises pyrolyzing a material made up or at least one pentene and containing only pentenes at a temperature between about 900 and MW 0., and under'a pressure corresponding to about '10 to 60 mm./Hg, in the absence of a catalyst, until substantially all of the pentene material has been converted to other substances and then distilling toseparate the pentadiene from the other conversion products. 5

2. A method of preparing. pentadienes which comprises pyrolyzing a material made up solely of at least one pentene at a temperature between about 900-l100 C., under a pressure corresponding to about 30 to 60 mm./Hg, in the absence of a catalyst for a period of 0.03 to 0.5 seconds to convert all of the pentene material to other subastances having boiling points substantially difierwas supplied by a 12" combustion furnace and the reaction tube was placed inside a V8" Pythagoras tube. The temperature was measured and a thermocouple placed between the inner and the outer tube in the center of the heated zone, The following results were obtained.

1 v A G. isoprene/100. Reaction tube Temp. az g ggz entering 0. GJmin. Quartz l, 100 l. 13 25. 7( Allegheny metal 900 3.-65 l7. 5

The results given above are not strictly comparable since-the yield of 25.7 grams obtained in the quartz tube represents all of the conjugated diolefins produced, the analysis being made by the maleic anhydride method. On the other hand, the yield of 17.5 grams obtained in the Allegheny metal tube represents isoprene, the total conjugated dioleflns by the maleic anhydride method being 38.5 grams in this instance.

-By properly adjusting the temperature and rate of flow of trimethyl ethylene, isoprene of 9095% purity can readily be obtained.

While there has been described above the preferred embodiment of the invention, it will be apparent that the method as outlined may be modified in various particulars without departing from the scope of the invention. The temperature may be varied from the range given depending on the particular. pentene which is being pyrolyzed' and depending on whether a product is desired which gives little trouble in separating the diolefin from the remaining prod-' ing from that of the pentene constituents of the Y material treated and then distilling to separate the pentadienes from the other conversion products.

3. A method of preparing isoprene which comprises pyrolyzing a material constituted solely by a branched chain pentene at a temperature between about 900 and 1100 C., and under a pressure corresponding to 10 to mm/Hg, in the absence of acatalyst, until substantially all of the pentene has been converted to other substances and then distilling to separate the isoprene.

- 4. A method of preparing isoprene which comprises pyrolyzing a" material constituted solely by a branched chain pentene at a temperature between about 900 and 1100 C., and under a pressure corresponding to about 30 to 60 mm./Hg, in the absence of a catalyst, until substantially all of the pentene has been converted to other substances and then distilling to separate the isoprene. 1'

5. A method of preparing' isoprene which comprises pyrolyzing a material constituted solely by a branched chain pentene at a temperature between about 900 and 1100 C. in the absence of a'catalyst under a pressure corresponding to between about 10 to 60 mm./Hg until substantially all of the pentene has been converted to other substances and then distilling to separate the isorene.

6. A method of preparing isoprene which comprises subjecting trimethylethylene to a temperature between about 900 and 1100 C. in the absome of a catalyst under a pressure corresponding to between about 30 to 6G mm./Hg until substantially all of the trimethylethylene has been converted to other substances and then distilling to separate the isoprene.

7. A method of preparing isoprene which comprises subjecting isopropylethylene to a temperature between about 900 and 1100" C. in the absence of a catalyst under a pressure corresponding to between about 30 to 60 mmJHg until substantially all of the isopropylethylene has been converted to other substances and then distilling to separate the isoprene.

8. A method of preparing isoprene which comprises subjectingunsym. methylethylethylene to a temperature between about 900 and ll00 Cuin the absence or a catalyst under a pressure corresponding to between about 30 to 60 mum/Hg until substantially all of the unsym. methylethylethyl ene has been converted to other substances and then distilling to separate the isoprene.

JOY G. LICHTY. 

